Draft: Stop horizontal scrolling due to padding in Dialogs

Now when using the preferredWidth, we take into account the padding of the contentItem, similar to what we're doing with height already.

This is most obvious with the new flatpak KCM which has a small PromptDialog, which has horizontal scrolling without this patch:

I need to track down more instances of PromptDialog and Dialog to see if this somehow regresses something else.